Harnessing
the force of falling water may be the world's oldest source of mechanical power. Hydropower
currently supplies 10 percent of the nation's electricity and 80 percent of the electricity
now produced from renewable resources.

Normally, rain water and melting snow
flows by gravity, producing streams, rivers, and lakes. Hydropower facilities
intercept the water on its downward path, converting its mechanical energy
into electricity. Because the cycle of water evaporating from the heat of the
sun and falling back to earth is continuously renewed by the sun's energy,
hydropower is often considered a renewable energy resource. However, the
construction and operation of hydropower dams impact natural river systems and
fish and wildlife. Whether specific hydropower projects create unacceptable
environmental damage requires a case-by-case review.

There are several
types of hydropower facilities:

"Storage" projects impound water behind a dam, forming a
reservoir. Water is released through turbine-generators to produce electricity.
The water storage and release cycles can be relatively short, for instance,
storing water at night for daytime power generation. Or, the cycles can be
long, storing spring runoff for generation in the summer when air conditioner
use increases power demand. Some projects operate on multi-year cycles
carrying over water in a wet year to offset the effects of dry years.

"Run-of-river" projects
typically use relatively low dams where the amount of water running through
the powerhouse is determined by the water flowing in the river. Because
these plants generally do not hold back water behind storage dams, they tend
to affect upstream water levels and downstream stream flow less than storage
projects. Electricity generation from these plants will vary with changes in
the amount of water flowing in the river.

"Pumped-storage"
projects use off-peak electricity to pump water from a lower
reservoir to an upper reservoir. During periods of high electrical demand,
the water is released back to the lower reservoir to generate electricity.
There are only about 40 pumped-storage facilities in the U.S., but some are
very large. (Note: the Power Scorecard rates electricity from pumped storage
on the basis of the electricity used to pump the water and the impacts of the
storage operations.)

What are the environmental impacts?

It is the dams and
powerhouse operations essential to hydropower plants that cause the primary
environmental impacts. The changes in river conditions and the land and
vegetation bordering the water bodies caused by dams and powerhouse turbines
may impact fish populations and other wildlife significantly. Even small dams
can cause big impacts on to the health of regional fish populations. The
impacts of large dams are wide-ranging. The impacts of any dam depend upon
many important factors, including the location of the dam, the facility design,
the sensitivity of the local environment to effects of the hydropower facility,
and steps taken to modify the design and/or operation of each facility to
reduce potential impacts.

Many impacts (see list
below) can be significantly reduced by changing operations of the dam. For
example, installing fish passage systems can reduce impacts on migratory fish;
and converting a dam from peaking to "run-of-river" operation can ensure the
natural flow of the river remains undisturbed and can adapt the hydropower
facility to the unique conditions of each river system.

Because every river and every dam are different, the type and severity of impacts
caused by each dam differs. Because these potential impacts are severe, it is
important to distinguish the plants that have successfully reduced or eliminated
specific impacts from those that have not.

Since 1987 the licensing and review process conducted by the Federal government
has more thoroughly addressed environmental impacts. Before 1987 the environmental
impacts of facilities were considered inconsistently and sometimes not at all.
The Power Scorecard recognizes this change in the quality of environmental review
by giving a better environmental rating to projects reviewed since January 1987.

Recently the Low Impact
Hydropower Institute has created a Low Impact Hydropower Certification program
to identify and reward efforts by dam owners to minimize the impacts of their
hydropower dams. The program certifies hydropower facilities with impacts that
are low compared to other hydropower facilities based on eight environmental
criteria:

river flows

water quality

fish passage and protection

watershed protection

threatened & endangered species protection

cultural resource protection

recreation

facilities recommended for removal

The Power Scorecard recognizes plants that have obtained this
"low impact certification" by giving them a better environmental rating.

The following paragraphs
outline some of the kinds of environmental impacts hydropower plants
can create and measures that can be used to mitigate such impacts. The scope
and severity of such impacts vary from facility to facility, and depend on
site conditions and the extent to which possible mitigation measures are
actually used.

Potential environmental impacts

Hydroelectric facilities
disrupt natural river flows.By diverting
water out of the river for power, dams remove water needed for healthy in-stream
ecosystems. Stretches below dams may be completely de-watered. By withholding
and then releasing water to generate power for peak demand periods, dams may
cause downstream stretches to alternate between no water and powerful surges
that erode soil and vegetation, and flood or strand wildlife. These irregular
releases destroy natural seasonal flow variations that trigger natural growth
and reproduction cycles in many species. Peaking power operations can also cause
can cause dramatic changes in reservoir water levels - up to 40 feet - that
can degrade shorelines and disturb fisheries, waterfowl, and bottom?dwelling
organisms.

Dams also slow down the
flow of the river. Many fish species, such as salmon, depend on steady flows
to flush them downriver early in their life and guide them upstream years
later to spawn. Slow reservoir pools disorient migrating fish and significantly
increase the duration of their migration.

These impacts can, at
times, be mitigated by technological and operational enhancements to the
hydro project - e.g., minimum flow turbines, re-regulating weirs, and pulsed
operation at peak efficiency. Impoundments can be managed to create new
upstream and downstream habitat for fish species and to provide minimum
discharges and improved habitat during seasonal or annual drought conditions.

Hydropower may alter
river and riverside habitat.
Construction of a dam can flood riverside lands, destroying riparian and upland
habitats. Construction of a dam can also convert river habitat into a lake-like
reservoir, threatening native populations of fish and other wildlife. Warm,
slow moving reservoirs favor predators of naturally occurring species. Dramatic
changes in reservoir water levels, described above, can degrade shorelines and
disturb fisheries, waterfowl, and bottom-dwelling organisms.

Dams alter water quality.
Impoundments can cause changes and variation in temperature or the amount of
dissolved gases in the river.

Surface temperatures
in the reservoir may rise when the flow of the water is slowed. If water is
released from the top of the dam, this warmer water may increase river water
temperature down stream. Cooler downstream temperatures may result when cool
water is released from the bottom of a reservoir. Such altered conditions can
affect the habitat, growth rate, or even the survival of fish and other
species.

For hydropower projects
with intakes located deep in the reservoir, water with low dissolved oxygen
(DO) levels released to the river downstream may harm aquatic habitat in the
river and contribute to other water quality problems. Applying mitigating
technologies can improve dissolved oxygen levels.

Water sometimes passes
over a spillway, rather than through the turbines. As water plunges into the
pool at the base of the dam, too much air can be trapped in the water,
creating "gas supersaturation," a condition that in some fish species fosters
something called lethal gas bubble disease. This can be mitigated by
installing structures to keep fish away from such areas.

A dam or a powerhouse
can be a significant obstacle to fish migration.
Ladders or lifts can be installed to pass certain fish species upstream, though
multiple dams on a river reduces the success rate of these fish passage devices.
Fish migrating downstream can become disoriented, bruised, stressed, or mortally
injured from contact with turbines or other parts of the facility. Bypass systems
can improve survival rates for migrating juveniles. When fish are trucked or
barged around the dams, they may experience increased stress and disease and
decreased homing instincts. Survival rates for fish passing through large turbines
vary but may approach 90-95 percent. In the case of multiple dams along a river
these effects can significantly harm migrating populations of important, sensitive
juvenile fish populations.

Impoundments also slow
down the flow velocities of rivers. Slow reservoir pools may disorient
migrating fish, increase the duration of their migration, which in turn may
increase their mortality rate.

The steep decline in
salmon populations in the Pacific Northwest and California is perhaps the
best known negative environmental impact associated with hydroelectric
facilities. Although several factors have affected this decline - including
commercial fish harvests, habitat degradation, and artificial fish hatcheries -
hydropower dams have contributed significantly. The causes for these declines
and the best strategies for restoring these important fisheries are currently
the subject of a major public policy debate.

Hydropower projects can impede
the natural flow of sediments.
Flowing water transports sediment. When the flow velocities are reduced in an
impoundment, sediment drops out and collects on river and reservoir bottoms,
where it can affect habitat for fish spawning. The loss of sediment downstream
can degrade in stream habitat and cause the loss of beach at the mouth of the
river. The deposited sediment also may contain chemical or industrial residues
from upstream sources. Dams may block and concentrate contaminated sediment
in the impoundment. Dredging is used in some cases, though it is costly and
may raise questions regarding disposal of the dredged material. Various flushing
and piping techniques are available for moving non-contaminated sediment downstream.